Print this pageBasin-Range Extensional Architecture: Mormon Mountains Region, Nevada
CARPENTER, JAMES A. and DANIEL CARPENTER
A Shallow 
Dip
(5 to 25 degree) normal fault model was applied to the area
based on a geologic map covering part of the Mormon Mountains. Proponents
maintain that the range is an incipient metamorphic core complex. Klippen of
Paleozoic carbonate rock are interpreted as hanging wall remnants of a rooted
Mormon Peak low-angle normal fault ("detachment"). The Shallow Dip model was
applied elsewhere in the region such that the continental crust is purported to
be comprised of an imbricate stack of slab-like extensional allochthons
separated by 5 to 25 degree west dipping faults having dip separations exceeding
20 kilometers, and with regional extension greater than 100%. Serious problems
have been identified with this model.
New geophysically-based structural analyses suggest the crustal architecture
is dominated by high-angle normal faults dipping 60 plus or minus 10 degrees in
the upper 5-10 km. Regional extension is approx. 20%. The steep fault geometry
was determined using reflection seismic (utilizing diffractions) and borehole
data. Bouguer gravity gradients require modeling steep fault dip. Fault dips
range from 55 to 68 degrees in piedmont exposures. Structural analyses
initiating the use of seismic, gravity, and subsurface well data in the region
led us to a High-angle (60 plus or minus 10 degrees) normal fault model and an
enigma: Evidence was not found to support the existence of the purportedly
rooted low-angle normal faults. They are not imaged on the seismic data (e.g.,
where they would be required to cut reflectors) and not penetrated, as the
Shallow Dip model predicts, where deep subsurface well control exists.
Subsequent research by the U.S.G.S. also integrated geophysical and field data.
The High-angle Fault model was corroborated, and serious concerns were echoed
about the Shallow Dip model.
The disparate High-angle and Shallow-dip normal fault models cannot coexist
in 3-D space. The Shallow-dip model is based solely on interpreting klippen in
the Mormon Mountains, Tule Springs Hills, and Beaver Dam Mountains as remnants
of extensional allochthons. Down-slope vergence and subcrop field relations of
the klippen exhibit evidence of rootless gravity sliding. Many klippe lie upon
Quaternary clastics. Gravity slide detachment systems form as rootless surficial
features. This explains why they do not continue in the subsurface - thus
resolving the enigma of why they are not seismically imaged or drilled. Thus,
the High-angle Fault model is integral to the first-order extensional
architecture of the crust in the region, and the Shallow Dip
model is obviated
because it is based on the misinterpretation of rootless gravity slide systems.